The Cathode Ray Tube (CRT) has long been used as a video display, for example, in television sets and in computer display terminals. CRTs utilize an electron gun to selectively scan and energize a phosphor screen. The energized portions of the screen momentarily luminesce to provide a visual image. CRTs have a substantial depth, in order to accommodate the relatively large apparatus of the electron gun.
Electroluminescent display panels have been developed to provide a relatively thin display which does not have the size constraints inherent in the apparatus of a CRT. Electroluminescent display panels employ a matrix of phosphor pixels which are selectively fluoresced to form an image. The phosphor pixels of an electroluminescent display are caused to fluoresce by the direct application of electrical energy.
The electroluminescent display has a plurality of anodes and cathodes which are arranged in overlapping relation to form columns and rows of a matrix of pixel elements. An electroluminescent phosphor is disposed adjacent to each crossover point of the electrodes of the matrix. When a line and column electrode are simultaneously energized, the phosphor pixel element at the crossover point of the electrodes is caused to luminesce. An image is formed on the display by sequentially energizing rows of electrodes of the matrix and selectively energizing corresponding column electrodes.
The brightness of the display is dependent upon the voltage difference between energized row and column electrodes. Thus, increasing the voltage difference between row and column electrodes has the desirable effect of increasing the brightness of energized pixels. However, when row and column voltages are increased to provide added brightness, contrast of the image is sharply reduced when either a row or column energization voltage exceeds a characteristic "forming voltage" for the display. A desirable increase in brightness for the display has therefore not been achievable in view of this sharp loss of contrast with increasing voltage.
Accordingly, it is an object of the invention to provide a direct current electroluminescent display panel and associated driver circuitry which provide a substantially brighter image, without a corresponding sharp loss in image contrast.
A further object of the invention is to provide drive circuitry which defines an increased voltage difference for maximizing the brightness of energized pixels and defines a reduced voltage below the forming voltage for de-energized background pixels.
Electroluminescent display panels are most efficiently and economically constructed with a sharply defined area for displaying the video information. It is desirable to display such information in standard 40-column, 80-column or graphic display formats. If different display formats are mixed on a screen of data, the sharply defined display area of the electroluminescent panel may cause the display characters at the ends of lines to disappear from the screen. This problem is particularly likely to occur if the EL display panel is receiving data from a device, for example a computer, which operates with CRTs that have a less sharply defined display field. Under said circumstances, an end character of a line can be lost if the display is switched from either a 40-column or graphic display mode to an 80-column display mode within one screen of data.
Accordingly, it is an object of the invention to provide an interface circuit for an electroluminescent panel which synchronizes a clock generator to incoming data signals in order to provide a complete, left justified display of information when display modes are changed in one screen of data.
A further object of the invention is to provide such an interface circuit which checks the data mode for each line of the display and adjusts the timing of the interface circuitry to ensure that all data is displayed within the sharply defined display field of the screen.
These and other objects of the invention will become apparent from a review of the specification which follows and of the drawings which are described hereafter.